Infrared Spectra Of The Ch3nh3pbi3 Hybrid Perovskite: Signatures Of Phase Transitions And Of Organic Cation Dynamics

The high-resolution terahertz reflection and mid- and near-infrared transmission spectroscopic studies of CH3NH3PbI3 hybrid perovskite single crystals are performed for the temperature range from 5 to 350 K. The observation of the torsional mode of the molecular cation (at 306 cm(-1)) and of 12 other low-frequency modes not observed before is reported. Mode Gruneisen parameters for the strongest inorganic-cage modes are negative, which points to their interaction with a soft mode. Sharp peculiarities and the hysteresis take place in the multiphonon spectra at the phase transition from the tetragonal to the orthorhombic phase (similar to 160 K). Moreover, pronounced spectral changes are observed at similar to 70 K; they correlate with the transition to tunneling dynamics. A spectral line splitting at this temperature is tentatively attributed to the tunneling splitting.